First of all, what are your PC specs?
Low tier:
Tekkit
Low-mid tier:
SkyFactory 2.5
Direwolf20 1.7.10
Mid tier:
Direwolf20 1.12.2
FTB OceanBlock
SkyFactory 3
RealCraft
InfinityEvolved
StoneBlock2
FTB Continuum
PixelmonReforged
Mid-high tier:
FTB Revelation
SkyFactory 4
OmniFactory
Interactions
ProjectOzone3
MC Eternal
GT NewHorizons
DDSS
RAD
High tier:
Basically anything
Don't get me wrong, i'm not saying you shouldn't play lower tier modpack, I'm only implying you need "that" tier of raw computer power to comfortably run this specific modpack. Basicaly becase some modpack would require you to build base of specific size and complexity to actually "complete" modpack.
Since MY computer is not that strong, i could only play up to MID tier, and i will tell you my "All Time Favourite" modpacks.
FTB Infinity Evolved
This is my most favourite modpack, because of it's mild complexity. You will need to learn atleast basics of every mod in this modpack to get to endgame.
SkyFactory 2.5
I just love the modpack where you can get everything out of nothing, if you work hard enough. And this modpack give you ability to get OP, but you need to walk some miles in void beforehand. Start with tree, end among gods.
Direwolf20 1.7.10
This is first modpack i played on this comunity servers, it have place in my heart, tho... it's an OLD modpack. Kinda relaxing. Nothing special.